Pressurized nitrogen to extrude molten steel-silicon alloy

ABSTRACT

It has been found that pressurized nitrogen may be satisfactorily employed to impart the extrusion pressure required when extruding the melts of steel-silicon alloys to form fine diameter wire. In previous practice with other alloys of steel, such as steel-aluminum alloy, it was necessary to use relatively expensive inert pressurized gases, such as argon and helium. Nitrogen could not be used with these systems in that reaction with the melt caused orifice plugging nitride precipitates to form.

United States Patent Rakestraw PRESSURIZED NITROGEN TO EXTRUDE MOLTEN STEEL-SILICON ALLOY June 17, 1975 3,658,979 4/1972 Dunn ct al. 164/66 X Primary Examiner-R. Spencer Annear Attorney, Agent, or Firm-Russell E. Weinkauf 57 ABSTRACT lt has been found that pressurized nitrogen may be satisfactorily employed to impart the extrusion pressure required when extruding the melts of steel-silicon alloys to form fine diameter wire. In previous practice with other alloys of steel, such as steel-aluminum alloy, it was necessary to use relatively expensive inert pressurized gases, such as argon and helium. Nitrogen could not be used with these systems in that reaction with the melt caused orifice plugging nitride precipitates to form.

2 Claims, I Drawing Figure PATENTEDJUN 17 ms 3 889L739 PRESSURIZED NITROGEN TO EXTRUDE MOLTEN STEEL-SILICON ALLOY This invention relates generally to methods wherein fine diameter wire is produced directly from the melt of various metals and metal alloys. More specifically, the invention is directed to an improvement in the process economics for producing wire from steel-silicon alloys by a continuous extrusion of the melt.

In producing fine diameter wire according to the afore-mentioned procedures, the metallic melt is extruded through a small orifice as a continuous molten stream and into an oxygen-containing medium. When the hot jet issuing from the extrusion orifice contacts the oxygen-containing atmosphere an instantaneous reaction takes place which results in the formation of an oxide film about the peripheral surface of the jet. The film called the stabilizing film prevents surface tension break-up of the liquid stream until it becomes solidified by cooling.

Some metals, for example, aluminum, can be processed per se to yield fine diameter wire because their oxides are stable and insoluble in the melt a necessary condition to form the stabilizing film. On the other hand, the oxide of iron does not have this property. Consequently, before steel wire can be successfully produced in accordance with this process, it becomes necessary to include as part of the melt a second alloying metal whose oxide is stable and insoluble in the molten alloy. Generally, the second metal concentration will range from between about 0.5 to 5.0 percent on the weight of the alloy, with from 1 to 2 percent being most commonly used. Metals which have either been employed or suggested for this purpose include aluminum, magnesium, beryllium, chromium, lanthanum, titanium and silicon. Aluminum has been most widely used in the past. However, in more recent process development efforts, silicon has been most favored.

When extruding molten steel alloys to form wire, a positive head pressure must be provided to drive the melt through the orifice capillary at ejection rates which are sufficient for forming a free-streaming jet. This is accomplished by maintaining a supply of pressurized gas over the melt pool above the extrusion orifree. The gas or gases employed for this purpose must not react with the melt to form melt insoluble materials which can occlude or obstruct the orifice capillary. Ob viously, processing cannot continue when this happens. In order to avoid the problem of precipitate formation and eventual orifice plugging, it has been necessary in prior practice to employ either argon or helium as pressurizing gases. Since these inert gases are relatively expensive, their use increases the over-all processing costs. Since a great deal of attention has been devoted to reducing processing costs, there has been considerable interest in finding operable but less expensive gases which could be used to provide the head pressure needed for extrusion.

It is, therefore, an object of this invention to improve the process economics for producing fine diameter wire directly from the melt of an alloy of steel.

It is a further object of this invention to provide a relatively inexpensive but fully satisfactory pressurizing gas which may be used in extruding the melts of a steelsilicon alloy to produce fine diameter wire.

It has now been discovered that nitrogen may be used to provide the required extrusion pressure when melt extruding steel-silicon alloys to produce a wire product. This is possible because it has been found that silicon nitride inclusions will not form in the melt even at very high nitrogen pressures unless the silicon concentration of the melt is in excess of 23 percent by weight. Since the amount of silicon present in steel alloys used to fabricate wire by direct extrusion of the melt rarely exceeds 2 percent by weight of the alloy, the likelihood of such inclusions forming is practically non-existant.

It had been thought in the earlier work involving the processing of alloys such as steel-aluminum and steeltitanium that extrusion pressures could be provided only by inert gases such as argon and helium when forming wire from the melts of steel alloys. As a matter of fact, when employing pressurized nitrogen with aluminum and titanium-containing steels, the nitrides of aluminum and titanium do precipitate and form orifice plugging inclusions. As has been noted, it has now been found that this does not happen when processing steelsilicon alloys and nitrogen can be used as a pressurizing gas. The advantage is apparent when one considers that nitrogen is approximately seven times less costly than argon and 40 times less than helium.

For a description of a type apparatus which may be employed in practicing this invention, reference is now made to the drawing. The FIGURE- represents a schematic cross-sectional view of a typical apparatus for extruding the melt of a steel-silicon alloy as a free streaming jet to form fine diameter wire. The molten metal charge is contained in crucible 2, having a base plate 3, with the crucible and base plate being supported by pedestal 4. Insulating cylinder 5 and susceptor 6 enclose the crucible 2 and its base plate 3. The heat required for conducting the process is provided by induction heating coils 7. An extrusion head pressure is provided by nitrogen under pressure supplied through gas line 8, which communicates with the interior of the unit through the unit head 9. Sealing rings 10 serve to maintain the pressure within the enclosure and prevent leakage past base plate 3. The molten metal 1 is forced through orifice 11 in orifice plate 12 by the applied head pressure to form a filamentary shaped molten jet. Upon emerging from orifice 11, the nascent jet passes through a film-forming atmosphere contained within the cavity 14 of pedestal 4. The film-stabilized molten jet then passes through a cooling column (not shown) where sufficient heat is removed for conversion to the solid state.

The following example illustrates a production run made in accordance with the invention as described hereinabove.

EXAMPLE An extrusion apparatus of the type illustrated in the FIGURE was employed to form fine diameter wire by extruding the melt of steel alloyed with 1.5 percent by weight of silicon. The crucible, which was brought to a temperature of about 1,530C. by induction heating, was filled with the molten metal alloy charge. Nitrogen gas under pressure was then supplied over the melt to provide a head pressure of up to 20 p.s.i.g. This produced a pressure gradient across the orifice plate which caused the molten steel alloy to be ejected through the capillary orifice and into a chamber below the orifice which was supplied with carbon monoxide as the filmtive head pressure is provided over the melt to force the molten alloy through an orifice and into a film-forming oxygen-containing atmosphere as a free-streaming jet wherein a stabilizing film is formed about the peripheral surface of said jet, the improvement which comprises: effecting said positive head pressure over the melt by means of nitrogen gas under pressure.

2. The method of claim 1, wherein the melt of said steel-silicon alloy contains from about 1.0 to 2.0 percent by weight of silicon. 

1. IN THE METHOD FOR PRODUCING FINE DIAMETER WIRE FROM THE MELT OF A STEEL-SILICON ALLOY HAVING A SILICON CONCENTRATION OF FROM ABOUT 0.5 TO 5.0 PERCENT BY WEIGHT BASED ON THE WEIGHT OF THE ALLOY, WHEREIN A POSITIVE HEAD PRESSURE IS PROVIDED OVER THE MELT TO FORCE THE MOLTEN ALLOY THROUGH AN ORIFICE AND INTO A FILM-FORMING OXYGEN-CONTAINING ATMOSPHERE AS A FREE STREAMING JET WHEREIN A STABILIZING FILM IS FORMED ABOUT THE PERIPHERAL SURFACE OF SAID JET, THE IMPROVEMENT WHICH COMPRISES: EFFECTING SAID POSITIVE HEAD PRESSURE OVER THE MELT BY MEANS OF NITROGEN GAS UNDER PRESSURE.
 2. The method of claim 1, wherein the melt of said steel-silicon alloy contains from about 1.0 to 2.0 percent by weight of silicon. 